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Authoring XML Elements  An XML element is made up of a start tag, an end tag, and data in between.  Example: Matthew Dunn  Example of another element with the same value: Matthew Dunn  XML tags are case-sensitive:  XML can abbreviate empty elements, for example: can be abbreviated to

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Authoring XML Elements (cont’d)  An attribute is a name-value pair separated by an equal sign (=).  Example: Emeryville  Attributes are used to attach additional, secondary information to an element.

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Authoring XML Documents (cont’d)  Authoring guidelines:  All elements must have an end tag.  All elements must be cleanly nested (overlapping elements are not allowed).  All attribute values must be enclosed in quotation marks.  Each document must have a unique first element, the root node.

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Authoring XML Data Islands  A data island is an XML document that exists within an HTML page.  The element marks the beginning of the data island, and its ID attribute provides a name that you can use to reference the data island.

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Document Type Definitions (DTD)  An XML document may have an optional DTD.  DTD serves as grammar for the underlying XML document, and it is part of XML language.  DTDs are somewhat unsatisfactory, but no consensus exists so far beyond the basic DTDs.  DTD has the form:

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DTD (cont’d) Occurrence Indicator: IndicatorOccurrence (no indicator)RequiredOne and only one ?OptionalNone or one *Optional, repeatable None, one, or more +Required, repeatable One or more

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XML Query Languages  The first XML query languages  LOREL (Stanford)  XQL  Several other query languages have been developed (e.g. UNQL, XPath)  XML-QL considered by W3C for standardization  Currently W3C is considering and working on a new query language: XQuery

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A Query Language for XML: XML-QL  Developed at AT&T labs  To extract data from the input XML data  Has variables to which data is bound and templates which show how the output XML data is to be constructed  Uses the XML syntax  Based on a where/construct syntax  Where combines from and where parts of SQL  Construct corresponds to SQL’s select

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Semistructured Data and Mediators  Semistructured data is often encountered in data exchange and integration  At the sources the data may be structured (e.g. from relational databases)  We model the data as semistructured to facilitate exchange and integration  Users see an integrated semistructured view that they can query  Queries are eventually reformulated into queries over the structured resources (e.g. SQL)  Only results need to be materialized

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What is a mediator ?  A complex software component that integrates and transforms data from one or several sources using a declarative specification  Two main contexts:  Data conversion: converts data between two different models  e.g. by translating data from a relational database into XML  Data integration: integrates data from different sources into a common view

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